Conventionally, in wire communication and wireless communication, improving the utilization efficiency of the frequency band is required given the increasing demand. In order to improve the use efficiency of a frequency band, for example, a technique has been disclosed that divides and transmits the spectrum of the transmission signal into a plurality of bands (hereinbelow referred to as “sub-spectra”), receives the sub-spectra that have been transmitted and restores them to the original modulated signal (refer to Non-Patent Document 1). In this technique, by utilizing the empty bands dispersed on the frequency axis, the bands that are not used are reduced. Furthermore, removing a portion of a sub-spectrum reduces the total of the occupied bandwidth of the signal. The technique disclosed in Non-Patent Document 1 realizes an improvement of the utilization efficiency of the band of frequencies in the above manner.
FIG. 16 is a function block diagram showing a function configuration of a communication system 500 that is implemented using a related technique. The communication system 500 includes a transmitting device 510 and a receiving device 520.
The transmitting device 510 performs transmission by dividing a transmission signal into a plurality of sub-spectra. The receiving device 520 receives the signal transmitted from the transmitting device 510 and restores the modulated signal prior to the division.
As shown in FIG. 16, the transmitting device 510 is provided with a modulation circuit 601 and a transmission filter bank 602, and a D/A converter 603. The receiving device 520 is provided with an A/D converter 611, a reception filter bank 612, and a demodulation circuit 613. The transmission filter bank 602 is provided with a series-parallel conversion circuit 604, a FFT (fast Fourier transform) circuit 605, a dividing circuit 606, N (where N is an integer of 1 or more) switches SW-1 to SW-N, N frequency shifters 607-1 to 607-N, an adding circuit 608, an IFFT (inverse fast Fourier transform) circuit 609, and a parallel-series conversion circuit 610. The reception filter bank 612 is provided with a series-parallel conversion circuit 614, an FFT circuit 615, an extraction circuit 616, N frequency shifters 617-1 to 617-N, a distortion compensating circuit 618, and adding circuit 619, an IFFT circuit 620, and a parallel-series conversion circuit 621.
The flow of a signal in the communication system 500 shall next be described. FIGS. 17 (A) to (C) are drawings that shows an example of processing when the transmitting device 510 divides the band into N parts (N=2) and arranges them by dispersion. FIGS. 17 (D) to (F) are drawings showing an example of processing when the receiving device 520 combines the bands that have been divided by the transmitting device 510. The modulation circuit 601 of the transmitting device 510 modulates the data signal to be transmitted by a method such as QPSK and inputs the modulated signal that has been waveform shaped as shown in (A) of FIG. 17 to the transmission filter bank 602. The output signal from the transmission filter bank 602 is converted to an analog signal by the D/A converter 603 and transmitted.
Processing is performed as follows in the transmission filter bank 602. First, the series-parallel conversion circuit 604 performs series-parallel conversion of the input signal, and the FFT circuit 605 performs a fast Fourier transform to convert the signal from the time domain to the frequency domain. Next, the dividing circuit 606 multiplies coefficients that divide the signal bands shown by the dotted lines 701-1 and 701-2 in (A) of FIG. 17 into N by the modulated signal that has been converted to the frequency domain, and generates N sub-spectra ((B) of FIG. 17). Next, the frequency shifters 607-1 to 607-N arrange the N sub-spectra by dispersing over predetermined bands on the frequency axis, and the adding circuit 608 sums the outputs of the frequency shifters 607-1 to 607-N ((C) of FIG. 17).
Next, the IFFT circuit 609 performs a fast inverse Fourier transform to convert the signal from the frequency domain to the time domain. Then, the parallel-series conversion circuit 610 performs parallel-series conversion. At this time, regarding bands that are to be partially deleted, prior to input to the frequency shifters 607-1 to 607-N, conveyance of the signal is blocked by putting those switches SW-1 to SW-N corresponding to the deletion in the open state (OFF). Thereby, due to the signal components not being placed in the relevant bands, it is possible to perform transmission in a state of a portion of the spectrum having been removed. Accordingly, it is possible to remove the frequency band required for transmission.
The A/D converter 611 of the receiving device 520 converts the received signal to a digital signal, and inputs the post-conversion digital signal to the reception filter bank 612. The demodulation circuit 613 demodulates the modulated signal outputted from the reception filter bank 612, and restores the data signal.
Processing is performed as follows in the reception filter bank 612. First, the series-parallel conversion circuit 614 performs series-parallel conversion of the input signal, and the FFT circuit 615 performs a fast Fourier transform to convert the signal from the time domain to the frequency domain. Next, the extraction circuit 616 multiplies coefficients shown by the dotted lines 701-3 and 701-4 in (D) of FIG. 17 by the received signal that has been converted to the frequency domain, and extracts N sub-spectra. Next, the frequency shifters 617-1 to 617-N return the sub-spectra that have been extracted to their respective bands prior to be shifted by the frequency shifters 607-1 to 607-N of the transmitting device 510 ((E) of FIG. 17). Next, the adding circuit 619 adds together all of the sub-spectra and obtains the combined modulated signal ((F) of FIG. 17).
Next, the IFFT circuit 629 performs a fast inverse Fourier transform to convert the signal from the frequency domain to the time domain. Then, the parallel-series conversion circuit 621 performs parallel-series conversion. At this time, the transmission signal is not received in the receiving device 520 for the some bands of which the spectrum was removed in the transmitting device 510. For this reason, some kind of compensation processing is required. For example, not only is there not a component of the transmission signal in this band, but there may exist a noise component that causes degradation of the reception characteristic. Therefore, the distortion compensation circuit 618 performs compensation by inputting a value based on the sub-spectrum that was received by the receiving device 520 to the band in which the signal was transmitted in the transmitting device 510, and inputting “0” for a band in which the signal was removed in the transmitting device 510. Thereby, the noise component in the band is removed for the band in which the signal was removed in the transmitting device 510, and the reception characteristic can be improved.
In the above manner, the communication system 500 divides the occupied band of a transmitted signal, and arranges each sub-spectrum that is generated by dispersion in arbitrary locations on the frequency axis. For that reason, discontinuous empty bands can be effectively utilized. Also, by not transmitting some bands of a transmission signal spectrum, the frequency bandwidth that is required for transmission is reduced, and it is possible to improve frequency utilization efficiency.